This invention is in the field of photoflash lamps of the so-called high-voltage type, which contain a primer material connected across inlead wires, there being no conventional filament in such a lamp. These lamps further contain a hermetically sealed light-transmitting envelope made of glass at the present time along with a combustion-supporting gas such as oxygen together with a loosely distributed filling of a suitable light-producing combustible material such as shredded foil of zirconium, aluminum or hafnium, for example, which upon ignition produces a high intensity flash of actinic light.
Electrically actuated photoflash lamps may be classified generally into two voltage types: low-voltage and high-voltage. The low-voltage types usually are intended to be flashed by a battery, or a charged capacitor, having a voltage of about 1.5 volts to 15 volts, whereas the high-voltage flashlamps are intended to be flashed by a firing pulse of a few hundred volts or greater such as can be produced by striking a piezoelectric material. Conventional low-voltage flashlamps contain a filament connected across inlead wires. When the filament is heated by a firing current, it ignites a primer material which in turn ignites a combustible material such as metal foil which, with the aid of oxygen in the lamp, produces a flash of light. In typical high-voltage flashlamp constructions, the primer material is connected directly across and between a pair of inlead wires extending into the lamp envelope. The primer material may be positioned and carried in the lamp on top of a glass or ceramic insulating member through which the inlead wires extend, or may be carried in a cavity provided in such member. In a different construction, the primer material is carried on or in a depression in the inner wall of the envelope at the bottom of the lamp. In still a different high-voltage flashlamp construction, disclosed in U.S. Pat. Nos. 2,868,003 and 3,000,200, both to Warren Albrecht, the primer material is applied to one or both of the inlead wires within the lamp and the electrical circuit is completed through the combustible shredded metal foil in the lamp.
Various primer materials suitable for the ignition means in the above type lamp constructions are known. The known primer materials generally consist of a solid mixture of a readily combustible fuel such as phosphorous or combustible incandescible metal with an oxidizer compound for the fuel such as alkali metal and alkaline earth metal chlorates as well as perchlorates, including sodium perchlorate, potassium perchlorate, barium chlorate, sodium chlorate, and potassium chlorate. The fuel in the primer material is generally a powdered incandescible metal such as zirconium, hafnium, thorium, aluminum, magnesium, boron, silicon or other alloys which upon actuation by a high-voltage pulse ignites the filamentary combustible material. The known primer materials are generally prepared as a liquid suspension in an organic or aqueous solvent to provide an adherent mass of the primer material between the spaced apart inleads. A known organic liquid suspension for the primer material that is described in U.S. Pat. No. 2,972,937 to C. G. Suits utilizes nitrocellulose as the binder agent and it requires careful handling during storage and lamp manufacture to avoid accidental ignition. A less sensitive aqueous suspension of the primer material utilizing the conventional ignition mixture employs hydroxyethylcellulose or methylcellulose, polyvinyl alcohol, or polyvinyl pyrrolidone as water-soluble binder agents.
A number of important considerations for the primer material to reliably ignite the combustible material in a high-voltage flashlamp of the type above generally described are pointed out in my aforementioned copending patent application. An important operating characteristic of the primer material is the voltage at which the primer's electrical resistance breaks down and decreases thus allowing sufficient electrical energy to be transferred from the firing pulse at its available voltage level to cause primer ignition. Excessive variation in this breakdown voltage characteristic could lead to a failure of the lamp to flash or to flash with an inferior light output. As regards this operating characteristic, it is also desirable for the initial breakdown voltage level to be sufficiently high so that the primer material is not overly sensitive to accidental ignition from electrostatic charges which can build up during ordinary handling of the flashlamps. On the other hand, the breakdown voltage level in the primer material should not be raised in a manner which also impairs reliable ignition when the firing pulse is applied across the spaced-apart inleads.